Mobile communication terminal apparatus are required to communicate with base stations in many directions. The direction of a base station seen from a mobile communication terminal apparatus varies if the terminal apparatus moves. Because of that, a non-directional antenna is generally mounted on the mobile communication terminal apparatus. Non-directional antennas make it possible to have communications with the other party of any direction, but, in general, they underperform on communications in a direction towards which the directivity of directional antennas are turned. Recently, there has been a demand for improvement in communication performance, and techniques for that were developed and are now put to practical use.
For example, there is a technique of improving communication performance by controlling the directivity of a directional antenna towards the direction of arrival of signals.
In one configuration for directional antennas having controllable directivity, there is a plurality of antenna elements being arranged, and the phase and amplitude of a signal which is fed to each antenna element are controlled depending on antenna elements. The directivity of a directional antenna is given by the synthesis of radio waves emitted from each antenna element. Directivity control may be done by analogally or digitally controlling the amplitude and phase of a feeding signal. An ADC (Analog to Digital Converter) is used to digitalize feeding signals.
In another configuration for directional antennas having controllable directivity, the length of a non-radiation element of a Yagi-Uda antenna which has a radiation element and a plurality of non-radiation elements is varied electrically. See Reactive Controlled Directive Arrays (ROGERF. HARRINGTON) IEEE Transactions on antennas and propagation, vol. AP26, No. 3, May 1978, p 390-395 (Document 1).
Directional antennas described in Document 1 are configured in a manner in which non-radiation elements are arranged in a circular shape, with the radiation element at the center thereof. In order to control directivity, the reactance value of each non-radiation element is varied to thus change the length of an electrical element in each non-radiation element. By virtue of this, the directivity of the Yagi-Uda antenna may be optionally controlled. ESPAR antennas using this system are disclosed in Japanese Patent Application Publication No. 2001-024431 (Document 2).
In addition, there is a method which simplifies the configuration or the control by limiting the reactance value of non-radiation elements of the Yagi-Uda antenna having controllable directivity, as described above, to two states: an OPEN state and a SHORT state. See SWITCHED PARASITIC ANTENNAS FOR CELLULAR COMMUNICATIONS THIEL DAVID V. SMITH STEPHANIE/ARTECH HOUSE 2002 (Document 3). Antennas using this method are disclosed in Japanese Patent Application Publication No. 2001-36337 (Document 4), Japanese Patent Application Publication No. 2001-345633 (Document 5), and Japanese Patent Application Publication No. 2003-258533 (Document 6).
Furthermore, there are MIMO (Multi Input Multi Output) techniques that improve communication performance by transmitting/receiving signals via multiple antennas. In MIMO system, each antenna element is required to have little relationship with the others. A transmission apparatus transmits a plurality of data streams at the same time using a plurality of antenna elements. A receiving apparatus processes signals which were received by a plurality of antenna elements to thus recover the original plural data streams. This MIMO is an effective technique particularly under the multipath environment.